[petsc-users] ILU and Block Gauss Seidel smoothing

Barry Smith bsmith at mcs.anl.gov
Mon Jul 18 13:14:08 CDT 2011 

On Jul 18, 2011, at 9:43 AM, Gianluca Meneghello wrote:

> Barry,
> 
> thanks again for your input. That would be really good.
> 
> When executing my program with the options you suggested I obtain
> various errors (unknown pc etc) so I tried to figure out how to avoid
> them. Is it something like this what you intended?
> 
> ./main
> -mg_pc_type composite
> -mg_pc_composite_pcs ilu,ilu
> -mg_ksp_type preonly

    I don't think you want this one. It says not to use a Krylov method on the outside; the preconditioner will just apply the two ILU triangular and that is it.  In other words it won't even solve the linear system. You likely want -mg_ksp_type gmres

   Barry

> -mg_sub_0_pc_factor_mat_ordering_type FESOrder
> -mg_sub_1_pc_factor_mat_ordering_type FWSOrder
> -mg_pc_composite_type multiplicative
> -mg_ksp_view
> 
> provided that
> 
> KSPSetOptionsPrefix(ksp,"mg_");
> 
> has been applied to the KSP I want to modify.
> 
> Thanks again
> 
> Gianluca
> 
> 
> On 13 July 2011 15:33, Barry Smith <bsmith at mcs.anl.gov> wrote:
>> 
>> On Jul 13, 2011, at 4:02 AM, Gianluca Meneghello wrote:
>> 
>>> Thanks Barry.
>>> 
>>> What I'm doing at the moment in order to pass the ordering to the PC is
>>> 
>>> PCFactorSetMatOrderingType(PC,"FESOrder");
>>> 
>>> where the FESOrder(...) function computes the ordering and has been
>>> registered in the main function with
>>> 
>>> MatOrderingRegister("FESOrder",0,"FESOrder",FESOrder);
>>> 
>>> The same is done for al the different orderings I'm using inside the
>>> PCSHELL suggested by Jed.
>>> 
>>> My understanding is that the FESOrder(...) function is evaluated every
>>> time the PCApply(...) function is called.
>> 
>>   Definitely not. Looking at PCSetUp_ILU()
>> 
>>  } else {
>>    if (!pc->setupcalled) {
>>      /* first time in so compute reordering and symbolic factorization */
>>      ierr = MatGetOrdering(pc->pmat,((PC_Factor*)ilu)->ordering,&ilu->row,&ilu->col);CHKERRQ(ierr);
>> 
>> it will actually call the ordering routine a TOTAL of ONE TIME if you use SAME_NONZERO_PATTERN to KSPSetOperators() or PCSetOperators(). (it is smart enough to reuse the ordering).
>> 
>>   BTW: I don't think you even need to use a PCSHELL preconditioner. You can get the same effect with a PCCOMPOSITE.  Just create a single KSP in the usual way and use the options
>> 
>> -pc_type composite -pc_composite_type ilu,ilu -sub_0_ksp_type preonly -sub_1_ksp_type preonly -sub_0_pc_factor_mat_ordering_type FESOrder -sub_1_pc_factor_mat_ordering_type anotherorder somethingelse
>> 
>> 
>>   Barry
>> 
>> 
>> 
>> 
>> 
>>> 
>>> If there was a way to store the IS containing the ordering — or any
>>> other array — into the shell context (this I know how to do that) and
>>> the pass the IS directly to the preconditioner instead of using
>>> PCFactorSetMatOrderingType would be the best solution for me.
>>> 
>>> Thanks
>>> 
>>> Gianluca
>>> 
>>> 
>>> On 12 July 2011 20:15, Barry Smith <bsmith at mcs.anl.gov> wrote:
>>>> 
>>>> On Jul 12, 2011, at 3:47 AM, Gianluca Meneghello wrote:
>>>> 
>>>>> Jed, thanks for your answer.
>>>>> 
>>>>> This is exactly what I was looking for (it took me some time to test it...).
>>>>> 
>>>>> Let me ask you a couple of additional questions:
>>>>> 
>>>>> 1) in this configuration, is there an alternative way to pass the
>>>>> ordering to the preconditioner? Becuse my ordering depends on the grid
>>>>> an not on the solution, it is constant during all the computations and
>>>>> I have no advantage on calling the function that creates multiple
>>>>> times (the pc is built and destroyed several times).
>>>> 
>>>>   Not sure exactly what you want here. You can put the ordering information in the shell context and just keep it there forever and don't need to to recompute.
>>>> 
>>>>    PCShellSet/GetContext()?
>>>>> 
>>>>> 2) what's the best way to make the shell pc selectable from the option
>>>>> database, so that it can be replaced with more standard ones?
>>>>> 
>>>> 
>>>>        /* here the user can provide -pc_type shell or ilu or whatever */
>>>>        KSPSetFromOptions(ksp);
>>>>        KSPGetPC(ksp,&pc);
>>>>        /* the following lines will only do something if the user selected the PCType of shell otherwise they are ignored */
>>>> 
>>>>        PCShellSetContext(pc,....)
>>>>        PCShellSetSetUp(pc,....)
>>>>        PCShellSetApply(pc,.....)
>>>> 
>>>> 
>>>>   Barry
>>>> 
>>>> 
>>>>> Thanks
>>>>> 
>>>>> Gianluca
>>>>> 
>>>>> On 1 July 2011 14:43, Jed Brown <jed at 59a2.org> wrote:
>>>>>> On Fri, Jul 1, 2011 at 07:11, Gianluca Meneghello <gianmail at gmail.com>
>>>>>> wrote:
>>>>>>> 
>>>>>>> I've tried to implement Jed's solution as it was looking faster to
>>>>>>> try. This is what I came up with for two different orders, FENOrder
>>>>>>> and FWNOrder respectively
>>>>>>> 
>>>>>>> 283       KSP ksp;
>>>>>>> 284       PC pc;
>>>>>>> 285
>>>>>>> 286       ierr = KSPCreate(PETSC_COMM_WORLD,&ksp);  CHKERRQ(ierr);
>>>>>>> 287       ierr = KSPSetOptionsPrefix(ksp,"mg_");    CHKERRQ(ierr);
>>>>>>> 288       ierr = KSPSetOperators(ksp,J,J,SAME_NONZERO_PATTERN);
>>>>>>> CHKERRQ(ierr);
>>>>>>> 289       ierr = KSPSetFromOptions(ksp);    CHKERRQ(ierr);
>>>>>>> 290       ierr = KSPGetPC(ksp,&pc);         CHKERRQ(ierr);
>>>>>>> 291       ierr = PCFactorSetFill(pc,1);     CHKERRQ(ierr);
>>>>>>> 292       ierr = PCFactorSetMatOrderingType(pc,"FENOrder"); CHKERRQ(ierr);
>>>>>>> 293       ierr = KSPSolve(ksp,b,x); CHKERRQ(ierr);
>>>>>>> 294       ierr = KSPDestroy(&ksp);  CHKERRQ(ierr);
>>>>>>> 295
>>>>>>> 296       ierr = KSPCreate(PETSC_COMM_WORLD,&ksp);  CHKERRQ(ierr);
>>>>>>> 297       ierr = KSPSetOptionsPrefix(ksp,"mg_");    CHKERRQ(ierr);
>>>>>>> 298       ierr = KSPSetOperators(ksp,J,J,SAME_NONZERO_PATTERN);
>>>>>>> CHKERRQ(ierr);
>>>>>>> 299       ierr = KSPSetFromOptions(ksp);    CHKERRQ(ierr);
>>>>>>> 300       ierr = KSPGetPC(ksp,&pc);         CHKERRQ(ierr);
>>>>>>> 301       ierr = PCFactorSetFill(pc,1);     CHKERRQ(ierr);
>>>>>>> 302       ierr = PCFactorSetMatOrderingType(pc,"FWNOrder"); CHKERRQ(ierr);
>>>>>>> 303       ierr = KSPSolve(ksp,b,x); CHKERRQ(ierr);
>>>>>>> 304       ierr = KSPDestroy(&ksp);  CHKERRQ(ierr);
>>>>>> 
>>>>>> It looks like you are putting all of this in PCApply_YourShell(). I suggest
>>>>>> making
>>>>>> struct YourShellContext {
>>>>>>   PC fenpc,fwnpc;
>>>>>>   Vec work;
>>>>>>   PetscBool issetup;
>>>>>> };
>>>>>> Then pass a pointer to this thing in when you create the PCShell. Then
>>>>>> something like
>>>>>> PCSetUp_YourShell(PC pc) {
>>>>>> struct YourShellContext *ctx;
>>>>>> Mat A,B;
>>>>>> MatStructure mstr;
>>>>>> const char *prefix;
>>>>>> char iprefix[256];
>>>>>> PCShellGetContext(pc,&ctx);
>>>>>> PCGetOptionsPrefix(pc,&prefix);
>>>>>> if (!ctx->issetup) {
>>>>>>   ierr = MatGetVecs(A,&ctx->work,PETSC_NULL);
>>>>>>   PCCreate(comm,&ctx->fenpc);
>>>>>>   PCSetType(ctx->fenpc,PCILU);
>>>>>>   PCFactorSetFill(ctx->fenpc,1);
>>>>>>   PCFactorSetMatOrderingType(ctx->fenpc,"FENOrder");
>>>>>>   snprintf(iprefix,sizeof iprefix,"%sfen",prefix);
>>>>>>   PCSetOptionsPrefix(ctx->fenpc,iprefix);
>>>>>>   PCSetFromOptions(ctx->fenpc);
>>>>>>   /* Same as above for ctx->fwnpc */
>>>>>> }
>>>>>> PCGetOperators(pc,&A,&B,&mstr);
>>>>>> PCSetOperators(ctx->fenpc,A,B,mstr);
>>>>>> PCSetOperators(ctx->fwnpc,A,B,mstr);
>>>>>> PCSetUp(ctx->fenpc);
>>>>>> PCSetUp(ctx->fwnpc);
>>>>>> ctx->issetup = PETSC_TRUE;
>>>>>> }
>>>>>> Then in PCApply_YourShell(PC pc,Vec X,Vec Y) {
>>>>>> ...
>>>>>> PCGetOperators(pc,&A,PETSC_NULL,PETSC_NULL);
>>>>>> PCApply(ctx->fenpc,X,Y); /* apply first ordering */
>>>>>> VecScale(Y,-1.0);
>>>>>> MatMultAdd(A,Y,X,ctx->work); /* Compute fresh residual */
>>>>>> PCApply(ctx->fwnpc,ctx->work,Y);
>>>>>> }
>>>>>>> 
>>>>>>> I don't know if this is what you intended... is there a way to make
>>>>>>> KSP recompute the factorization with the different ordering without
>>>>>>> destroying and recreating the KSP?
>>>>>> 
>>>>>> Store both as in the code above.
>>>>>> 
>>>>>>> 
>>>>>>> Concerning Barry's option, js it possible to use the PETSc ilu
>>>>>>> factorization function (and eventually the ones provided by external
>>>>>>> libraries) inside my PCSHELL, passing it the various ordering I will
>>>>>>> need? If so, what's the best way to access it?
>>>>>> 
>>>>>> Does the code above answer this question?
>>>>> 
>>>>> 
>>>>> 
>>>>> --
>>>>> "[Je pense que] l'homme est un monde qui vaut des fois les mondes et
>>>>> que les plus ardentes ambitions sont celles qui ont eu l'orgueil de
>>>>> l'Anonymat" -- Non omnibus, sed mihi et tibi
>>>>> Amedeo Modigliani
>>>> 
>>>> 
>>> 
>>> 
>>> 
>>> --
>>> "[Je pense que] l'homme est un monde qui vaut des fois les mondes et
>>> que les plus ardentes ambitions sont celles qui ont eu l'orgueil de
>>> l'Anonymat" -- Non omnibus, sed mihi et tibi
>>> Amedeo Modigliani
>> 
>> 
> 
> 
> 
> -- 
> "[Je pense que] l'homme est un monde qui vaut des fois les mondes et
> que les plus ardentes ambitions sont celles qui ont eu l'orgueil de
> l'Anonymat" -- Non omnibus, sed mihi et tibi
> Amedeo Modigliani

More information about the petsc-users mailing list